It may be useful in various situations to model the operation of engines (e.g., gasoline or diesel engines) in order to appropriately identify, understand and/or respond to various engine states. For example, it may be useful to model pressures, temperatures, mass flow rates, and various other parameters, with respect to various points or volumes within an engine in order to inform the adjustment of operating characteristics of the engine and thereby improve power delivery, fuel efficiency, engine life, and so on. Similar information regarding engine operation may similarly be gleaned, in certain settings, from appropriate instrumentation of the engine (e.g., by providing the engine with various pressure, temperature, mass flow, and/or other sensors). Such instrumentation can be expensive, however, particularly with respect to mass-produced consumer or industrial vehicles. Instrumentation is also sometimes prone to malfunction or other failure, which may lead to imperfect identification or understanding of current engine operating states and/or require time-consuming and expensive repairs.
Various models are known that may be utilized to represent the operating states of various engine locations and/or components or groups of components. For example, physics based models and regression models may be utilized to predict residual mass and volumetric efficiency of an engine cylinder, compressor and turbine maps may be utilized to predict characteristics of compressor and turbine operation, the compressible gas flow equation may be utilized to predict characteristics of throttle or valve flow, and so on. Many of these models are highly non-linear and their implementation may accordingly be complex and/or computationally intensive.
One type of engine model, an air system model, utilizes a mean value model to represent engine cylinders. Under this type of model, flow through an engine may be assumed to be continuous, as opposed to exhibiting discrete cylinder events. As such, using intake mixture density (e.g., from intake air and recirculated exhaust gas), engine speed, engine displacement, and a volumetric efficiency correction, the flow of mass through the engine may be calculated.
Prior manifestations of air system models, however, tend to exhibit numerical instability in certain situations. For example, when resistance to mass flow is very low (e.g., when relevant throttles and/or valves are fully or nearly fully open), small changes in pressure can result in large changes in mass flow. In turn, such changes in mass flow may significantly affect the modeled pressure, resulting in unstable oscillations in the predicted pressures that may not accurately reflect actual engine operating conditions. Past efforts to address this issue have included introducing additional resistance to the mass flow in certain circumstances (e.g., at pressure ratios close to 1) and using a very small time step (e.g., less than 1 ms) for the model. Such efforts, however, may tend both to contribute to steady state model errors and to require an excessive number of calculations for a given time interval. The latter issue, in particular, may prevent use of such models with respect to real-time operation of an engine because the high demand for calculations tends to consume significant portions, including potentially all, of the processor capacity of the available computer resources (e.g., the engine control unit associated with a particular engine).
A method and system for modeling engine operation as an air system while addressing these and other issues is accordingly needed.